ACU-M™ Improving In-Building Communications

Application Note: AN-2306-2

ACU-M™ Improving In-Building Communications

Purpose

This application note will describe methods in which the Raytheon’s ACU-M can be used to improve in-building communications. The application note will discuss permanent and temporary methods at which different devices can help increase the ability to transmit or receive land mobile radio communications from within a building, below-grade, or behind obstructions.

Introduction

Land mobile radios, whether used in vehicles or as handheld portables, are an important tool used everyday by first responders to make their jobs safer and more efficient. In the most part, these radio systems function as designed, and serve the end-user with reliable communications. However, when the duties of a first responder require them to enter a building, or operate belowgrade during emergencies, the ability of their radios to communicate to the base, incident command or dispatcher may become an issue. One physical constraint of land mobile radio communications is its inability to transmit and receive radio waves through obstructions such as buildings or below-grade structures. The failure of a land mobile radio to transmit and receive communications from within a building or below-grade has cursed radio users since the beginning of land mobile radio communications (see Figure 1).

Figure 1: Blockage or Absorption of Low-Power Handheld Radio Transmission

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Solutions

Land mobile radios were first introduced to public safety, in the late 20’s, in the form of shortwave receivers mounted inside patrol vehicles. The earlier radio systems, used by the Detroit Police Department allowed only one-way transmissions from the headquarters to the patrol vehicle. It was never certain that the officer actually received the transmission. As technology progressed, communications allowed the officer to converse with the dispatcher in a two-way, half-duplex fashion. This occurred in the 30’s with the Bayonne Police Department. Officers were now able to communication to the dispatcher, headquarters and the other officers in the field. The natural progression was to make the mobile radio, portable, hence the handie-talkie or handheld land mobile radio. Officers and firefighters were now able to leave their vehicles and communicate with their dispatchers, commanders and amongst themselves with small radios that they can carry on their belts. One drawback of the handheld radio is its lack of transmit power (1 to 5 watts) to link to the main repeater of the radio system. Figure 2 shows that the mobile radio mounted inside a vehicle has a distinct advantage because it can usually transmit at much higher power levels (5 to 50 watts, or greater) than the portable radio, thus mobile mounted radios are more likely to link with the radio system’s repeater, even through obstructions, such as the building, below.

Figure 2: High-Power Mobile is able to Penetrate Building Material

Technologies such as receiver voters and comparators, multicast and simulcast are techniques used in today’s modern radio systems that help compensate for the performance of the lower powered handheld radios. These techniques leveled the playing field, making handheld radios just as important to the first responder, as their mobile radios inside their patrol cars or apparatus. However, when an officer or firefighter enters a structure he may be faced with the uncertainty that his handheld portable radio may not be able to reach the main repeater site or satellite receiver sites due to blockage as a result of the materials of the building surrounding him.

Some agencies have migrated from lower frequency radios (VHF) to higher frequency radios (UHF, 800 MHz) to take advantage of the improved “penetrating” properties of the shorter wavelengths, in attempts to counteract the signal-attenuating properties of the building’s surface (see Figure 3). Although VHF transmissions can travel around very large obstructions and travel many more miles than 800 MHz transmissions, VHF is no match for dense urban locations, where reflection and absorption of VHF signals is prevalent.

Figure 3: Improve Penetration, Shorter Wavelength is Better

In the dense metro environment, more and more obstructions are blocking the desired line-of-site (LOS) between the handheld radio and the repeater, and even higher frequency radio systems are not the answer. Additionally, building materials such as concrete, foil-insulation, steel and Low-E-Glass (low-emission glass), which has a microscopically-thin coating of light blocking metal oxide, can completely isolate the radio user within a building.

Permanent Solutions – Permanent in-building repeater systems can be used to create an electrical pathway that will allow transmissions to be routed from the inside of the building, to the outside of the building, through electronic means, and visa versa. One method is to place a receiver and donor antenna outside of the building to retransmit the low-power handheld radio transmissions that are picked up by distributed coverage antennas throughout the inside of the building. Figure 4 illustrates transceivers, roof-top donor antenna, distributed interior coverage antennas and cables used in a permanent in-building repeater system. The low-power transmissions from the officer’s portable radio can circumvent the RF absorbing or RF blocking properties of the building material, and utilize an electrical pathway to the donor antenna on top of the building. Conversely, transmissions can be received by the donor antenna on the rooftop, amplified, and then routed to the numerous coverage antennas within the building, which then allows the portable radio inside the building to receive transmissions from outside. There exists similar inbuilding communication systems that distribute transmit and receive communications over fiber optic lines. In all cases, the transceivers used in these systems are called Bi-Directional

Amplifiers, BDA.

Figure 4: Bi-Directional Amplifiers, Permanent Solution

Instead of an array of transceivers, antennas and combiners, another method of signal distribution is to install a radiating cable or “leaky coax” along the vertical length of the building. The cable, with radiating slots along its length, would capture transmitted signals from the handhelds from within the building, and also leak transmitted signals receive from outside of the building, into the building from the donor receiver on the top of the building.

Figure 5: Radiating Cable, Permanent Solution

Figure 5 illustrates a permanent radiating cable solution that utilizes a BDA to facilitate the transceiver duties.

Although municipalities have started requiring new and existing commercial and government buildings of certain sizes and construction types to be equipped with permanent in-building repeater systems, there are many more municipalities unable to fund or put forth such directives to change their local building codes. These ill equipped buildings have become potential death traps to first responders and the occupants that they are sworn to protect.

Firegrounds Communications - Firefighters communicate using Hierarchal Communication system where the Incident Commander will communicate with several Team Leaders; and the Team Leader will communicate with the a 3 or 4 firefighter team inside the building. With this Social Communications Environment the Incident Commander will typically remain outside of the building relaying messages to the team leader inside of the building using land mobile radios. In some instances, the IC may enter the building using the floors below the incident as a command post. Whichever command structure is used at the firegrounds, this social method assures that no one person is responsible for too many tasks, the incident will remain modular, and since communications occurs in a hierarchal basis, this technique tends to reduce traffic and loading of the radio systems. Communications techniques used by firefighters are varied. Some rely on repeated systems, and some on a dispatch environment, where the IC communicates with the dispatcher, and the IC relays messages to the team leaders at the firegrounds. But once the firefighter teams enter a building or underground structure, many fire service agencies may have no choice, but to practice the use of simplex or tactical communications.

Figure 6: Firefighters Often Use a Dispatch Communications Hierarchy

In principle, simplex firegrounds communications is useful in an in-building incident because the radio communications using the main repeater between the incident commander, team leaders, and firefighters may not possible due to the RF blocking properties of the building’s surface. Figure 7 shows that the firefighters have switched their low-power handheld radios to a simplex firegrounds or tactical channel, and are now able to communicate in a localized fashion. The Incident Commander at the command post is also able to communicate with the personal inside

the building by utilizing a radio in a similar simplex tactical mode. These radios operating in simplex mode must all transmit and receive using the same frequency. Additionally, the incident command usually has access to another radio that will allow him to communicate with the dispatch center using half-duplex methods (utilizing the system’s repeater). This is important especially as the incident grows, the IC must be able to escalate the incident and request more companies or equipment at the firegrounds. It is not uncommon for the command post to have a number of radios performing different tasks.

Figure 7: Simplex Firegrounds Communications Does Not Rely on Repeater

A residual benefit of simplex firegrounds communications is “Frequency Reuse”. Since the radiating “footprint” of a low power handheld is rather small; the tactical channel can be “reused” at many spatially separated locations. Figure 8 illustrates frequency reuse of a tactical channel by three incidences that are not within the RF range of each other.

Figure 8: Several Simplex Firegrounds are Reusing the Same Frequency

Preferred by some fire agencies, utilizing the main repeater of the radios system allows the communications between the firefighters inside the building and the incident commander, as well, this arrangement allows the dispatcher to be in constant contact with all levels of command. If other buildings are involved in the vicinity, the other companies will be able to communicate in a collective fashion (see Figure 9).

Figure 9: Firegrounds Communications Utilizing the System Repeater

Although simplex firegrounds communications has become a common practice with fire service agencies, public safety communications, ideally, relies on repeated communications. This assures that each police officer in the field has a communications link between the other officers in the field, as will as the dispatcher, just in case the officer needs to request emergency assistance.

Figure 10: Building Material Blocks Communications Signals to the Outside

Portable Repeaters - Portable repeater receives weak simplex communication from inside the building, and then rebroadcast to the system’s main repeater with a radio that can transmit through the building material with additional transmit power, or by utilizing a remote located antenna close to the outside of the building. Figure 11 illustrates a basic portable repeater that contains one transceiver that interfaces with the system’s main repeater, and another transceiver that interfaces with the in-building simplex communications. The repeater controller decides which transceiver rebroadcasts the communications at a given moment. One of the advantages of a portable repeater is that it can be position directly within the building where the incident is taking place. The user can select the optimal position, which is a compromise between a good Line-of-Site (LOS) path to the radio system’s repeater, and the lowpower handheld radio users inside the building. When a portable repeater is placed inside a building, it needs to be positioned away from the core of the building, and preferably near the outer wall to afford a good link to the main system’s repeater. One disadvantage of the portable repeater is its weight can be rather heavy due to its integral battery. First responders will be burdened to carry another piece of heavy equipment into a building; along with the usual implements they carry to manage a fire. Portable repeaters also have an operating life cycle defined by the capacity of its battery.

Figure 11: Portable Repeater Rebroadcast Simplex Communications

Using our previous building fire example, Figure 12 illustrates how one or more portable repeaters can be tactically located. Remember, the antenna must be positioned such that it can obtain a satisfactory link to the radio system’s repeater. Note that the antenna of the portable repeater inside the subway tunnel has been extended above-ground. This is a necessity, but may be impractical due to the resistive loss of the additional length of the antenna cable. Excessively long antenna cables can reduce the transmit power and receive sensitivity of the portable repeater such that the repeater is no longer useful.

Figure 12: Portable Repeaters Located Inside Building, or Below-Grade

Vehicular Repeaters - Vehicular repeaters are an alternative to the portable repeaters. In essence, a vehicular repeater, which remains inside the vehicle, utilizes the more powerful transmitter of the mobile radio inside the vehicle, converting it into a secondary repeater. (see Figure 13). It is common practice to locate the vehicle close to the firegrounds or incident. Example “A” illustrates how a low-power simplex firegrounds radio inside a building can communicate to the half-duplex system through the system’s main repeater. This is a classic vehicle repeater application, also known as, Range Extenders. Example “B” illustrates how the battalion chief uses his low power radio to send a high-power transmission to the simplex firegrounds radio users in a high-rise building. Example “B” is a common method of using a vehicular repeater at a firegrounds incident.

Figure 13: Simple Vehicular Repeater Utilizes Mobile Radio and Handheld Radio

The biggest advantage of a vehicular repeater is that it does not need to be carried into the building; it can remain on the apparatus where power is not an issue (see Figure 14).

Figure 14: Vehicular Repeater Turns Radio inside of Battalion Chief Vehicle

Figure 15 illustrates how the battalion chief’s vehicle, which contains the vehicular repeater, is functioning as a secondary repeater site for the lower powered hand-held radios being used inside the building.

Figure 15: Vehicular Repeater inside Battalion Chief Vehicle Located Near Building

One important factor with regards to vehicular repeaters is that the vehicle supporting the repeater must be position in a tactical location that is relatively close to the building to assure a reasonable transmit and receive link to the low-power handheld radios within the building. In Figure 15, the vehicular repeater contains a low-power UHF radio used to link the battalion chief to the repeater. The vehicular repeater also contains a high-power VHF radio that will link the firefighters using simplex communications back to the battalion chief. Physics tells us that the transmissions of the simplex users inside the building may not be able to communicate back to the battalion chief. But, the link will make certain that commands from the battalion chief will be heard by the firefighters. Note that it would be imperative for the agency responsible for deploying temporary communication repeaters to perform periodic site surveys when dealing with in-building or belowgrade communications, or other structures that could prove difficult to public safety communications when a real incident occurs.

Advantages and Disadvantages – Portable and vehicular repeaters, both, have advantages and disadvantage. Understanding how the temporary repeater will be deployed for each specific incident will help determine which method is more practical. The following tables list some of the common advantages and disadvantages of each temporary repeater type:

Vehicular Repeater

Advantage

Disadvantage

Power is less of an issued. Power derived from vehicle No need to carry heave case into building Uses the radio already inside the vehicle

DDDDDDD

Less exposure to weather and elements Able to support unorthodox repeater functions Operator can remain in a safe location, inside vehicle Difficult to misplace, attached to vehicle Vehicle must be relatively close to building or structure Unable to easily support very tall buildings

DD

Portable Repeater

Power is self contained (battery), no power cords required Can be positioned to optimize LOS

Advantage

Disadvantage

DDD

Able to serve a very tall building (multi positioned assets) Configured in an adverse environment

DDDDDD

Can be misplaced Possible exposure to weather and elements Battery has a life cycle, discharge is an issue Must be carried into structure, heavy Must carry radios, antennas, cables, etc into structure

Dealing with Interference – Portable and vehicular repeaters are unique, in that they, unlike permanent repeater solutions, can behave in unpredictable manner once deployed at the incident. Not only does the physical reaction to the surrounding materials (buildings, cars, terrain) determines the performance of the temporary repeater, but the radiating properties of the radio and transceivers used with the temporary repeater will play an important factor in the overall success of the repeater.

Adjacent Channel Interference – One of the specifications that define a radio’s receiver is, Adjacent Channel Isolation. Adjacent channel isolation indicates how well a radio’s receiver rejects any extraneous signals that are just outside of the frequency or channel of interest. Naturally, we would only like to listen to a specific channel, and not hear conversations taking place on the adjacent channels. This is usually determined by the filtering used at the first mixer of the receiver. Figure 16 shows a properly designed radio receiver rejecting all adjacent transmitted frequencies.

Figure 16: Properly Designed Receiver Filtering of Adjacent Channels

But, temporary repeaters discussed in this application note are inherently susceptible to adjacent channel interference, because they may contain high-power transceivers, and their antennas are in close proximity to one another. Figure 17 illustrates how the signal from a high-power radio can affect another radio’s receiver that is within close proximity.